Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing the new design; and,

FIG. 2 is a front view of a second embodiment thereof.

The broken lines showing display screens, portions of the display screens, text, and graphical elements form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 